1. Field of the Invention
This invention relates to spectacle lenses, and, more particularly, to improvements in progressive lenses for the correction of presbyopia.
2. Description of the Prior Art
Presbyopia, or loss of accommodation of the eye with age, is classically treated by means of a spectacle fitted with bifocal or trifocal lenses. Alternatively, two pairs of single-vision spectacles, one for near vision and one for far vision (if necessary) may be prescribed. In recent years, the progressive addition spectacle lens has become an increasingly popular third approach to the management of presbyopia. Such lenses offer two distinct advantages over bifocals and trifocals: they provide a continuous range of accommodative powers, and they have no visible dividing lines.
But progressive lenses also have well-known disadvantages. In particular, the intermediate-power zone invariably exhibits unwanted lateral astigmatism and focusing error. In addition, such lenses usually exhibit skew distortion (loss of orthoscopy), as well as unwanted power and prismatic imbalance in binocular viewing. These aberrations are inherent (i.e., unavoidable in principle) and much effort has been expended in attempts to reduce or minimize their effect.
There is another defect of progressive lenses that is not often (if ever) mentioned. Most progressive lenses, despite the feature of progressively varying power, are designed along the lines of a standard trifocal. That is to say, the distance portion of the progressive surface is spherical and occupies the upper half of the lens, just like the distance portion of a solid-type (Executive) trifocal; the reading area, too, is spherical, and occupies a segment-shaped area separated some 15mm from the distance vision area. These spherical distance and near vision areas are connected by a progressive corridor (the midline of which being usually an umbilic), and the inherent aberrations of the lens are compressed into the areas laterally disposed to the progressive corridor and the reading area. Not only are these aberrations objectionably strong (because the area into which they are packed is small), but the transition between the distance and intermediate areas, and between the intermediate and reading areas, is marked by relatively sharp changes in all optical characteristics: mean power, astigmatism and prism. Thus, the visual field afforded by the typical progressive lens is by no means a smooth and continuous one; rather, it is divided into alternately clear and blurred areas. Lenses exhibiting these discontinuous optical characteristics may not be tolerated by some patients.
In summary, the following may be cited as principal goals in the design of a progressive lens:
1. Optically stable and aberration-free distance and near-viewing areas. PA0 2. Progressive corridor of useful width and convenient length. PA0 3. Minimized surface astigmatism. PA0 4. Minimized lateral power error. PA0 5. Minimized skew distortion. PA0 6. Minimized binocular power and astigmatic imbalance. PA0 7. Minimized binocular vertical prismatic imbalance. PA0 8. Visual continuity, i.e., smooth and continuous optical effect.
Unfortunately, it is not possible to satisfy all design goals simultaneously, and design compromises are inevitable. Many forms of compromise are possible, each leading to a new design with its own peculiar features.
The progressive lens described in U.S. Pat. No. 4,056,311 (Winthrop, assigned to American Optical, 1977) features a spherical distance portion occupying the entire upper half of the lens and a large spherical reading portion. Consequently, the astigmatism within the intermediate area is highly compressed and of non-negligible magnitude. Provision is made for the correction of orthoscopy in the peripheral portions of the intermediate area, but this feature results in an undesirable concentration of aberration at the boundary between the corrected and uncorrected areas. The layout of this design is similar to that of a trifocal, and consequently the design lacks visual continuity.
The design put forward in U.S. Pat. No. 4,307,945 (Kitchen and Rupp, assigned to Itek, 1981) also features a spherical distance portion comprising the upper half of the lens, a large spherical reading portion, and correction for orthoscopy in the peripheral portions of the intermediate area. The astigmatism adjacent to the progressive corridor is reduced below normally-expected values by permitting astigmatism to occur at the midline of the corridor itself; however, the astigmatism that remains to either side of the corridor is by no means negligible. Aberrations are highly concentrated at the boundary between those areas that are and are not corrected for orthoscopy. This design, conceptually similar to the one previously described, lacks visual continuity.
The progressive lens design described in U.S. Pat. No. 4,315,673 (Guilino and Barth, assigned to Rodenstock, 1982) is similar to the two previously described designs in that it has a large, almost spherical distance portion and a large, almost spherical reading portion. In this design, less emphasis is placed on the maintenance of orthoscopy than in the two previously described designs. This permits slightly lower values of astigmatism and enables the astigmatism to be distributed more uniformly than in the previous two designs. Despite these improvements, the design still emulates the trifocal and consequently lacks overall visual continuity.
In Canadian Pat. No. 583,087 (Davenport), U.S. Pat. No. 4,274,717 (Davenport) and Netherlands Published Application 71-07504 (Biessels), similar progressive lenses are disclosed in which the progressive surface is divided into the three traditional viewing zones, with a large, spherical distance portion in the upper half of the lens, a large, spherical reading portion in the lower half, and a meridional progressive corridor connecting the distance and reading portions. In the Davenport construction the progressive surface is generated by portions of a family of circles developed by passing an inclined plane of constant inclination through a multiplicity of spheres. The Biessels construction is virtually identical except that the inclined plane may have varying inclination. With either lens the aberration level is substantial.
The progressive lens disclosed in U.S. Pat. No. 4,514,061 (Winthrop) reduces the astigmatism level of the traditional three-viewing-zone lens to an optimally low level by uniformly distributing the aberration in the intermediate zone through application of the Dirichlet principle. But this lens, like the lenses previously described, exhibits significant aberration and lacks visual continuity.
Each of the preceding designs (except for the lens of Biessels) is optically symmetrical about the corridor meridian. To enable the eye to track comfortably down the progressive corridor, the corridor of each lens must be inclined about 9.degree. from the vertical when mounted in the frame. This, however, may lead to uncomfortable binocular inequity between the two lenses in off-axis viewing at the intermediate-power level. Some designs incorporate asymmetry about the corridor meridian in an effort to control these unwanted binocular effects.
In U.S. Pat. No. 3,785,724 (Maitenaz, assigned to Essilor, 1974), an asymmetric lens is described whose aim is to provide equal astigmatic effect binocularly at the intermediate and near power levels. However, the lens also features a spherical distance area comprising the upper half of the lens and a large spherical reading area. Consequently, although the astigmatic effects may be equalized binocularly, the magnitude of the astigmatism is objectionably strong. Moreover, the lens, being comprised of three distinct viewing zones in the manner of a trifocal, does not provide visual continuity.
The asymmetrical design disclosed in U.S. Pat. No. 4,606,622 (Fueter and Lahres, assigned to Zeiss, 1986) aims to reduce to tolerable values binocular prism imbalance between the two lenses. But this design, too, has an almost spherical distance portion comprising the upper half of the lens, and a large, almost spherical reading portion. Consequently, the astigmatism at the intermediate level reaches significant values. Moreover, such a design, for the reason noted previously, cannot provide visual continuity.
From U.S. Pat. No. 3,687,528 (Maitenaz, assigned to Societe des Lunetiers, 1972), it is known that the inherent astigmatism of progressive lenses can be reduced in magnitude by permitting it to extend into the peripheral areas of the distance portion. This reduction is attained at the price of introducing astigmatism and power error at the distance vision level. But the remaining astigmatism is by no means negligible. Moreover, despite the reduced levels of astigmatism, the structure of the design does not afford optimum visual continuity.
U.S Pat. No. 4,580,883 (Shinohara, assigned to Seiko, 1986) describes a progressive lens in which the progressive surface is again divided into the three traditional viewing zones: far, intermediate, and near. As in the design discussed in the previous paragraph, astigmatism is permitted in the peripheral portions of the far and near zones, and this feature helps to reduce the maximum astigmatism encountered in the intermediate zone. Additionally, the far and near vision zones are so constructed that orthoscopy is strictly maintained at all points of those two zones. But orthoscopy is not maintained within the intermediate zone. Consequently, this lens, consisting of three viewing zones alternately corrected and uncorrected for orthoscopy, does not fulfill the requirement of optical continuity.
U.S. Pat. No. 4,640,593 (Shinohara, assigned to Seiko, 1987) describes improvements to the designs mentioned in the previous paragraph when the refractive power of the far zone is strongly positive, as required for the strongly hypermetropic (far-sighted) patient. This involves the introduction of a "base curve factor" to compensate for oblique astigmatic effects incurred in off-axis viewing. It does not alter the basic design of three distinct viewing zones, alternately corrected and uncorrected for orthoscopy.
In U.S. Pat. No. 2,878,721 (Kanolt, assigned to Farrand, 1959), a lens is disclosed in which astigmatism levels have been reduced to relatively low values. This is achieved by distributing the astigmatism over the entire area of the lens. But the price paid is a heavy one: both the distance and near centers are objectionably astigmatic, and the power error at those levels is severe. Thus, while such a lens indeed displays visual continuity, too much has been sacrificed to attain it and such a lens would not be acceptable to a wearer.
Accordingly, it is the object of the invention to provide a progressive addition spectacle lens with the smoothest possible distribution of dioptric power and lowest possible level of unwanted astigmatism, with orthoscopy at least approximately preserved in the lateral margins of the lens, and which in all power zones satisfies realistic requirements on stability of power and binocular compatibility.